The present invention relates generally to a fabrication process for computer-generated holograms, a computer-generated hologram fabricated by that process, and a computer-generated hologram.
For the purpose of prevention of forgery or faking, it has so far been known that tradable coupons, credit cards or the like are provided with holograms. For such holograms, there is a computer-generated hologram (CGH) that is obtained by forming interference fringes on a given recording surface using computation by a computer (see Patent Publication 1).    Patent Publication 1: JP(A)2001-013858    Patent Publication 2: Patent No. 3810917    Patent Publication 3: JP(A)2000-214750    Patent Publication 4: JP(A)2002-72837    Patent Publication 5: JP(A)2005-215570    Patent Publication 6: JP(A)2004-309709    Patent Publication 7: JP(A)2004-264839    Non-Patent Publication 1:
A. W. Lohmann and D. P. Paris: “Binary Franuhofer Holograms, Generated by Computer”, Appl. Opt., 6, 10, pp. 1739-1748 (October 1967)    Non-Patent Publication 2:
Wai Hon Lee: “Sampled Fourier Transform Hologram Generated by Computer”, Appl. Opt., 9, 3, pp. 639-643 (March 1970)
However, the computer-generated hologram of Patent Publication 1 had a limited load on computation, but had a narrow viewing angle in the longitudinal direction. For instance, FIGS. 17 and 18 are illustrative of prior arts. FIG. 17 is a side view of the case where a computer-generated hologram 101 is irradiated with reconstructing illumination light 102a composed of monochromatic light. As shown in FIG. 17, diffracted light 103 occurring from the uppermost unit area is not diffracted in the direction of a viewing point E. For this reason, at the position of the viewing point E, a reconstructed image at the uppermost site of the computer-generated hologram 101 cannot be viewed, and the viewing angle in the longitudinal direction does not spread out either. On the other hand, FIG. 18 is illustrative of the case where a computer-generated hologram 101 is irradiated with reconstructing illumination light 102b composed of white light. As shown in FIG. 18, each unit area is only seen in monochrome irrespective of being irradiated with the reconstructing illumination light 102 composed of white light. For instance, of diffracted light rays 103 occurring from the uppermost unit area, a ray traveling toward a viewing point E is a blue component B of the incident white light 102b. For this reason, at the position of the viewing point E a reconstructed image at the uppermost site of the computer-generated hologram 101 is seen in blue light B, at the center site it is seen in green G, and at the lowermost site it is seen in red R (not shown).
It is here noted that one specific technique of using a point light source as a light source for object light from an original image thereby making sure a viewing angle in the vertical (longitudinal) direction is disclosed in Patent Publication 2. With the technique of Patent Publication 2, object light takes the form of a spherical wave spreading from the point light source on the original image so that the viewing angle in the vertical direction becomes wide. However, because the recording area on the hologram recording surface is limited in the vertical direction, there is a change in the vertical direction viewing angle depending on the depth position of an object.